Insulating materials having temperature-dependent electrical resistance or capacitance characteristics have long been extensively used in overheat sensing and control applications. Thus, by virtue of the invention of Spooner and Greenhalgh disclosed and claimed in U.S. Pat. No. 2,581,212 overheat protection for electric blankets and similar articles is provided for the use of such materials to afford the essential safety factor. In accordance with the teachings of that patent, the insulating material is operatively associated with switch means and is coextensive with the heating element so that when the temperature anywhere in the blanket exceeds a predetermined maximum, the blanket heating power supply is interrupted. Because this insulating material is not altered physically or otherwise irreversibly changed in so functioning, it is useful repeatedly for this purpose as it acts as a sort of electrical switch constantly monitoring the blanket operating temperature limit.
A variety of insulating materials are identified in the prior art as being suitable for such use. Those include in addition to the preferred Nylon polyamide resin of the aforesaid patent, polymeric organic materials such as polyvinyl chloride and cellulose esters containing additives imparting the desired electrical characteristics. In U.S. Pat. No. 2,745,944 to Price, still another kind of material for this same purpose, sulphur-cured butadiene-acrylonitrile elastomer is disclosed. That material and all the others of the prior art, however, are in one respect or another, less than what has been desired and general recognition of that fact has failed heretofore to result in a thermal-sensing insulating material approaching the ideal which would combine the best properties and characteristics of each of those, but would be free, at least to a large degree, from their major drawbacks which are relatively low levels and ratios of changes in impedance with temperature and, in the case of DC volume resistivity, high levels of volume resistivity and low ratios of changes in volume resistivity to temperature. In addition as in the case of Nylon resin, the effect of humidity shifts the levels of impedance resistivity to the extent that control circuits become a problem.
The practical significance of such shortcomings of prior art thermal-sensing materials is apparent from the commercial electric blanket experience. Thus when exposed to moisture, the Nylon insulation loses its desirable electric properties to a large extent in only an hour or two, and even though that insulation in an electric blanket is covered by a layer of polyethylene and an overlayer of polyvinyl chloride as described in the Spooner and Greenhalgh patent reference above, moisture-induced deterioration occurs at a substantial rate. It is for this reason that there have been a rash of early failures of electric blankets in regions of high humidity such as the Texas Gulf Coast. The possibility of limiting the consequences of this destructive effect of high moisture-content atmospheres by increasing the shut-off temperature level so as to prolong the useful life of the Nylon insulation is not attractive for the reason that the safety factor is thereby substantially diminished.
The catastrophic effect of the presence of free sulfur in thermal-sensing insulation on wires of the type used in electric blanket structures has been demonstrated in tests under normal operating conditions running only six hours to wire failure.